Accelerometer



Oct.' 16, 1945.

W. W. BENTLEY, JR

FIG.

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ACCELEROMETER Filed April 8, 1942 2 Sheets-Sheet l EN: 7-1. E KJ?.

Oct.' 16, 1945. .w. w. BENTLEY, JR

ACCELEROMETER Filed April 8, 1942 2 Sheets-Sheet 2 ro'f/ 72443 Patented Oct. 16, 1945 'raras PATENT oi-'Flc (Granted under the act oi March 3, 1883, as amended April 30, 1928; 370 0. G. .757)

2 Claims.

'I'he invention described herein may be manu'- facturednd used by or for the Goverment for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to acceleration measuring instruments and, in particular, to devices for measuring acceleration in vertical directions.

When an airplane is in night, the lifting forces which hold the plane suspended in the air on a Weight llf Gravityg the said total force, F, may be expressed by the formula wherein a is the vertical acceleration of the plane, Since, furthermore, the acceleration a may be expressed as y 1L, wherein n is a factor. the above formula forl the total lifting force may be written as In the above formula W, which, as mentioned above, represents the weight of the plane, is constant, so that F canbe determined by measuring the factor n.

The aircraft industry `has tried to develop measuring instruments, for measuring said factor n, which instruments are commonly known as accelerometers, and employed for use in connec tion with the testing of airplanes.

One of thesey instruments, known under the name Weaver accelerometer employs a plurality of plungers each of which rests upon a differently calibrated spring and tends to compress the respective spring by its own weight. To effect a measuring operation, the instrument is mounted in the plane to be tested so that variations in the vertical acceleration of the plane react P011 the instrument plungers thereby causing the latter to vary the compression of the spring pertaining thereto. Since the calibration of each spring inasmuch as the mass may be excorresponds to a diierent value of n, that spring Awhich prevents a predetermined movement of the plunger pertaining thereto indicates the respective n of the plane under test.

In order to -facilitate the movement of the plungers in the said Weaver device, an arrangement was provided known under the name De Port accelerozneter according to which the plungers were suspended on tension springs rather than to have them rest on compression springs.

However, these known devices have the great disadvantage that they can be used for drop tests only, i. e. when the measuring'plunger at the time of measurement is in a vertical position. A further drawback of these known devices consists in the 'relatively high friction between the plungers and the guiding means therefor. It may also be mentioned that it is impossible to properly align the plungers and their coil springs and to maintain them in this alignment.

These known devices have also the very important disadvantage that a recording disc is required for registering a measurement, which recording disc has to be replaced by another one for subsequent measurements; in othervwords, it is not possible to make continuous measurements and/or recordings of the changes in acceleration as they appear with the airplane in Eight. Also calibrated springs, as used in the above mentioned known instruments, are not reliable since they lose their elastic properties.

It is, therefore, an object of my invention to provide a measuring instrument which, although simple in construction, will overcome the abov mentioned drawbacks.

Another object of my invention consists in the provision of an accelerometer which will yield more accurate readings than heretofore possible.

It is a further object of my invention to provide an instrument for measuring the acceleration of an airplane while the latter is in night.

Itis another object to provide an accelerometer which, when used in connection with an aircraft, will allow continuous recordings of the acceleration or changes in acceleration as they occur dur- -ing the operation of the aircraft.

It is still another object to provide an accelerometer which will, instantaneously. produce a Figure 1 diagrammatically illustrates the principle of the invention.

Figure 2 is a diagrammatic illustration of a rst embodiment of the invention.

Figure 3 is a second embodimentl of my inventlon.

Figure 4 illustrates an altitude correction chart.

Figure 5 is a mechanism for adapting the readings of the accelerometer to variable altitudes.

Figure 6 shows the preferable location of my accelerometer in an airplane, while the latter is in flight.

Principle of the invention It is well known that a liquid in a more or less U-shaped tube. open at both ends, will always assume a position in which the level of the liquid in both arms of the tube will be equal. This is due to the fact that the liquid in each arm is subjected to the same pressure. However, if the pressure acting upon the liquid in one arm in changed, with regard to the pressure acting upon the liquid in the other arm, the' above mentioned condition of equilibrium is disturbed so that the liquid in one arm will be at a higher level than the liquid in the other arm. Assuming now that the pressure acting upon the liquid in both arms is again equalized, the liquid willlreturn to its previous position in which the levels of the liquid were the same in both arms. Since the pressure acting upon the liquid in both arms was'assumed to be equal, it is obvious that the restoring force for returning the liquid to its previous position must be the weight of the liquid column which is defined by the difference in level of the liquid in both arms. In other words, the said liquid column constitutes a source of potential energy tending to change the levels of the liquid in the two arms and this change can be brought about by subjecting the said liquid column to an unbalanced force. lThis principle has been made use of in the accelerometer of the invention.

Referring to Figure l, the same shows a U- shaped tube I which contains a liquid 2 of a known density r. In the particular instance, illustrated in Figure l, the liquid in the arm 3 is at a lower level than the liquid in the arm 4. This is due to the fact that the pressure acting upon the surface 5 of the liquid in arm 3 is higher than the pressure acting upon the surface 6 of the liquid in arm 4. The height ofthe column, which column is dened by the difference in level of the liquid in both arms 3 and 4, is called the head h. The pressure acting upon the surface 5 is equal to the pressure on the surface 6 plus the pressure exerted by the said column. The weight of the said column per unit of cross-sectional area can be expressed as hXTXZ. Designating the pressure on surface 6 as po and the pressure acting on surfaces 5 as p1, it will be clear from the foregoing that pi=po+h r.

Assuming now that the above mentioned liquid column is acted upon by an unbalanced force such as would be produced by acceleration, while arm 3 is sealed and arm 4 is open, the liquid in arm 3 will move up from the level 1 to the level 8, whereas the liquid in arm 4 drops from the level 9 to I II. The difference between the levels 1 and 8 equals the difference between levels 9 and I0 and this difference is designated X. In other words, liquid 2 has shifted in the tubei through a distance X, and in so doing has reduced the head h in arm 4 and ,raised the levelin arm 3,therefore, causing an increase in the pressure of the air in assen?? arm 8. Therefore, the liquid level reached in arm 3 above position 1 is a measure of the force exerted on head h, and thereby makes possible the determination of the acceleration to which said liquid column was subjected.

Mathematically, the above mentioned unbalanced force produced by accelerating the liquid 2 in tube I may be expressed as F=ma, wherein v m=mass of liquid 2 a=acceleration with which liquid 2 moved and since 'a=n y where n is a multiple factor; a is the acceleration of gravity, the unbalanced force may be expressed as F=m XnX a. Inasmuch as wherein K=Ratio of specific heats for constant pressure and constant volume (for gases).

V1 is the volume of gas between the sealed end of arm 3 and level 8.

Referring now to a practical embodiment of the invention, and to Figure 2 in particular, the device shown therein comprises a casing I2 having mounted therein a support I3 of any desired material, preferably rubber. Carried by the support I3 is a U-shaped tube I4, made of some material that is a non-conductor of electric current, and which is pressed against the support I3 by a pad I5. The pad I5 is pressed against the tube I4 by means of a bearing plate I6 and a rod I1 having a threaded upper portion I8. The threaded portion I8 passes through and threadedly engages a cover I9 which closes the casing I2 and may be connected thereto in any desired manner, preferably by means of a thread 20. The upper portion of the rod I1 has rigidly connected thereto a knurled knob 2l for actuating the rod I1 so as to press the same against the bearing plate I6 or to space it therefrom. The tube I4 is furthermore held in the casing yI2 by means of. a lateral support 22 and a further support 23.

' switch-arm 5 i.

The tube I4 contains a liquid 24, preferably mercury, and is closed at one end. Provision is made that a differential pressure acts upon the liquid 24. For instance. in the specific embodiment shown in Figure 2, a gas 25 at a predetermined pressure, is confined in the upper portion of the closed right-hand arm 26 of the tube i4, whereas, the left-hand arm 21 of the tube I4 is open to, atmospheric pressure. Due to the thusestablished differential pressure, the liquid 24 -will stand at a higher level in arm 21 than in arm 26.' Arranged in the tube I4, preferably at the lowest portion thereof, is a contact 28 which is connected to a terminal 29 on the outside of the casing I2. Similar contacts 3c, 3i, and 32v are provided in the arm 26 which are respectively connected with terminals 33, 34, and 35, likewise on the outside of the casing l2. The contacts 3U, 3l, and 32 are spaced from eachother in accordance with the desired range and degree of sensitivity of the instrument. While, in the embodiment of Figure 2, only three contacts are shown in arm 26, it is, of course, understood that the number of contacts may be varied in accordance with the desired range and accuracy.v It should, likewise, be pointed out that, for this particular arrangement'only, (i. e. when the material of tube ifi is a non-conductor) the contacts do not have to be insulated from the tube. If it should be decided advantageous to use an electric-current-conducting material, such as copper, for tube iii, the contacts can easily be insulated from the tube, and consequently, preserve the utility of the indicating means.

As will be seen from FigureZ, the instrument shown therein comprises a conductor 36, such as a wire, which extends into the arm 26, preferably so that it will at all times be contacted b'y the liquid 2li. However, it is to be understood that this is by no means a requirement, since the extent to which the Wire is'contacted by the common panel 68.

Operation of first embodiment Assuming that the device shown in Figure 2 is to be used for measuring acceleration, for instance, of an airplane. To this end, the device is mounted in an airplane, preferably at its center of gravity as indicated at 69 in Figure 6, in

' which the base of casing I2 ,is sh'own parallel to the thrust line of the airplane. However, it should be noted that also other positions 0f the instrument, relative to the thrust line of the airplane, are possible, and may even be desirable, without aiecting the proper operation of the instrument. If it is desired that the instrument always remain in one and the same position, irrespective of the position of the plane orl other device, the acceleration of which is to be measured,` the instrument may be suspended in a universal joint or in the manner of a gyro compass. When the instrument has been properly located in the airplane, in accordance with the desired measurements to b effected,`and a continuous recording of the measurements is desired, the operator closes the switch 56, thereby establishing a circuit which comprises current source 33, now'closed switch 44, lines 65 and 36, terminal 29, contact 28, mercury 22, conductor 36, terminal 31, line 38, oscillograph 46,.and line' supposing now that the ariplane is in flight and encounters an up-gust or positive acceleration,

40 it will be clear from the explanation set forth liquid 2t merely depends on the desired measure- A ment to be eiected. The wire 36 leads to a termi.. nal 31 on the outside of casing i2. The terminal 31 is connected by line 38 to one terminal 39 of an oscillograph d6, the other terminal of which di is connected through line 42 with one terminal of a current source 3. The other terminal ci current source 53 is connected to a switch is which in its turn is connected byline 45 with iine 56 leading to the terminal 29. The line 65 is also connected to one terminal of a current source 37, the other terminal of which is connected to line 136. Branching off from line 36 is a line 56 which comprises a switch 50 and a The switch-arm 5l is normally held in open position, for instance, by a spring (not shown) and is moved into closed position by solenoid 52 when the latter is energized. One end of solenoid 52 is connected by line 53 with the terminal 33, whereas the other end of solenoid 52 is connected by line 54 with the line 55. The line 55 comprising a -lamp 56 is connected, on one hand, to line 48 and, onthe other hand, to a switch-'arm 51 which, similar to Switch-arm 5l, is normally held in open position and moved into closed position by energization of solenoid 58. One end of solenoid 58 is connected by line 59 to the terminal 34, whereas the other'end of solenoid 58 is connected by line 60 to line 6l. The line 6I likewise comprises a lamp 62 and has one end connected to line 48 whereas the other end leads to switch-arm 63. The switch-arm 63 is similar to switch-arms 5l and`51, normally so that a higher current ows through the oscillograph. If the airplane runs into a downgust, thereby encountering a negative acceleration, the situation is of course reversed, i. e. the unimmersed length of the conductor 36 increases and the resistance of the circuit becomes greater.

These variations in resistance are continuously recorded by the oscillograph 40 in the same order as they appear and thereby present a picture of the accelerations encountered by the plane.

-In this connection, it may be noted that this principle can also be employed without the use of conductor 36. For instance, the conductor 36 could be completely omitted and the tube I6 be made of electric-current-conducting material, such as steel. In this instance, the liquid 2d could be any liquid having a conductivity higher than the material of which the tube i4 is made. The terminal 31 then will be connected to the upper end of the arm 26 of tube I4, while terminal 29 is preferably connected to the lowermost portion of tube i4, which portion corresponds to the location of contact 28 in Figure 2. The contacts 30, 3|, and 32 must, of course, v.be insulated, in this particular instance, in order not to impede their intended function whichwill be described in detail later.

It will now be clear that, similar to the operation described above, after closure of switch 44, a current will ow from the current source 43 to terminal 28, into the conducting liquid 24, and

the surrounding portion o i tube I4 up to the liquid level in arm 26, and from there through the remaining tube portion of the arm 28, to terminal 81, and through the oscillograph 48.

Since, as is well known, the electrical resistance varies inversely to the cross-sectional area of the conductor, it is obvious that the electrical resistance will decrease when the liquid level in in arm 26 rises due to a positive acceleration, so

that the mercury reaches contact 30, a current ilows from current source 41, through line 46,

terminal 29, contact 28, mercury 24, contact 88, terminal 83, line 53, solenoid 52, lines 54 and 55, lamp 58, and through line 48 back to source 41. This current energizes the solenoid 52 so that the latter moves the arm -5l into closed position. This establishes a holding circuit for the lamp 58 which holding circuit comprises source 41, line 48, now closed yswitch 50, now closed switch 5I, solenoid 52, lines 54 and 65, lamp 56, and line 48. In this way, the lamp 58 remains lighted even after the mercury level in arm 26 has resumed its initial position and thereby disengaged the contact 38. In other words, thev lamp 56 in its lighted condition indicates that during the measurement the mercury level in arml26 had reached the contact 30 which corresponds to a definite acceleration. f

If the level of the mercury in arm 28 reaches contact 8l, a current" will flow through solenoid 58 and lamp 82.11; a manner similar to that described in connection with the solenoid 52 and lamp 56. Similarly, current will ilow through solenoid 64 and lamp 61 when the mercury level in arm 26 reaches contact 32. It will, therefore, be clear that the lighted lamps indicate the various accelerations encountered during the measure ing operation, and since each lamp when lighted corresponds to a definite acceleration which may be marked on the panel 68, adjacent the corresponding lamp, the greatest encountered acceleration may immediately be read from the panel. It will, likewiseI be clear that the lamps 56, 82 and 61 are lighted successively, thereby giving a visual picture oi' the change in acceleration as it occurs. When the desired measurement has4 been. made, it isl merely necessary to open, and subsequently, to close switch 50 again for conditioning the instrument for a new measurement.

In case the measuring instrument, according to the invention, has the liquid 24 in one arm, for instance arm 21, subjected to atmospheric pressure. it will readily be seen that material changes in altitude will aiect the calibration of `theinstrument. In other words, a correction in the calibration will be necessary if the instrument is to be used in materially diierent altitudes. To take care of this correction, the calibration of the instrument may be made adjustable, for instance as shown in Figure 5. As will be seen therefrom, a plurality of drums 10, 1I, and 12 is provided, corresponding in number to the number of possible readings during one and the same set-up of the instrument. (In Figure 2, three possible readings corresponding to the three contacts 30, 3|, and 32) The drums 10, 1I, and 12 are connected to a common shaft 13 which also carries a drum 14 and is rotatable by a knob 15 which latter is locked in a desired position by any convenient means, such as a locking pin. The drum 14 is provided with markings indicating various altitudes, whereas 'the drums 10, 1i, and 12 are provided with markings indicating various accelerations corresponding to the respective altitudes marked on drum 14. In other words, each altitude marking on drum 14 corresponds to three acceleration markings on drums 10, 1l, and 12, respectively. Therefore, if the calibration of the instrument is to be adjusted for a measurement in a changed altitude, it is merely necessary to adjust the drum 14 to the respective altitude; the calibrations pertaining tothe said altitude will then become visible through the windows 16, 11 and 18 provided in panel 68.

Instead of providing the correcting device of Figure 5, use may be made oi a correction chart such as shown in Figure 4. This chart has plotted acceleration against altitudes as applied to the respective instruments, thereby indicating the correction to be eiiected in the calibration oi the instrument. For' instance, if the instrument shows the acceleration 4g at sea level, it should show, according to the chart of Figure 4, the acceleration 3g at an altitude of 17,000 feet at the lighting-up of one and the same lamp. In other words, one and the same lamp, when lighted, indicates 4a measured at sea level or 3g measured at an altitude of 17,000 feet. V

While the instrument, as described so far, is provided with electric means for indicating the effected measurement, it is tube understood that the instrument according to the invention is not limited to such electric means.

For instance, fluid pressure responsive means, such as a pressure gage may be used as shown in Figure 3. According to this gure, a gas under a predetermined pressure is entrapped between the end of arm 26A and the adjacent liquid level, while different gas pressure is exerted upon the surface of liquid in arm 21A. This last mentioned pressure is substantially maintained constant during the operation of the device by any desired means, such as a pressure responsive valve 19 in connection with an accumulator 80, whereas, the iirst mentioned pressure varies during the said operation. Any change in this differential pressure due to acceleration is measured and indicated by the pressure gage 8|. The gas lin the two arms may be of the same or of different composition.

I Want it to be understood that my invention is not limited to the particular construction shown in the drawings but embraces various embodiments as covered by the following claims.

Having described my invention, I claim:

l. In an apparatus for indicating vertical velocity variations, a supportsubject to vertical velocity variations including a tubular liquid contalner having an elongated vertically extending portion, a liquid column yieldably supported in said vertically extending portion subject to relative vertical movement thereof incident to said vertical velocity variations, a plurality of signal dev1ces successively operable by said liquid column upon relative vertical movement thereof in one direction in said container to indicate the degree of movement of said liquid, and releasable means flor maintaining said signal devices in operative i signaling position upon subsequent movement of the liquid column in said container in the opposite direction incident to'said vertical velocity variations.

2. In an apparatus for indicating variations in vertical velocities, a support subject to vertical velocity variations including a. tubular liquid container having anxelongated vertically extending tubular portion, an elongated electrically conductive liquid column yieldably supported in said vertically extending tubular portion with its upper end normally positioned at a predetermined disu tance from the upper end of said tubular portion,

a plurality of electrical contacts vinsulated from l" each other extending to the interior of said tubular portion, spaced longitudinally along the tubular portion between the said normal liquid level and the end of said tubular portion for successive electrical contact with said liquid upon relative 15 ly energized, and switch means common to said circuits for rendering said magnet means inoperative.

WILLIAM W. BENTLEY, JR. 

